Is there something like Observable in linux that can watch for changes?

Question

linux

Is there some kind of function in Linux so that you can monitor file changes, or better, immediately the result of some command?
For example, //small pseudo -code
with the users command we get all users authorized in the system
onChange(users, () -> showMessage("New user logged in \ or logged out"))
Something like this?

Answer 1

php.net/manual/en/function.inotify-add-watch.php

Answer 2

What?
What would you like? Monitor file changes or receive a message about user authorization?